Envron Environmental – Ozone Systems – Benefits for Water Treatment:
- Disinfection at rates much faster than Chlorine (E-Coli killed at low Ozone dosages).
- Inactivation of viruses.
- Removal of Iron and Manganese.
- Control of Tastes and Odors.
- Can be used for some pesticide removal in water depending on severity
- Oxidation of Organics and Inorganics
- Improves taste, appearance, quality and acceptability of drinking water.
Systems Are Now Available for Municipal, Well-Water and Domestic Use
Ozonation – What is it?
Ozone is one of the most powerful water treatment compounds available to systems managers today. It is a technology that has been in continual commercial use for over 100 years and has distinct properties that allow disinfection of even heavily compromised water streams.
With the 1996 reauthorization of the Safe Drinking Water Act, Ozone was named as among Abest available technology@ (BAT) for small system compliance to National Primary Drinking water Regulations as overseen by the US Environmental Protection Agency.
DISINFECTION TREATMENT TECHNOLOGIES LISTED IN THE U.S. ENVIRONMENT PROTECTION AGENCY=S SURFACE WATER TREATMENT RULE (SWTR)
Ozone Ozone is a powerful oxidant with high disinfectant capacity. A study found that within a pH range of 6 to 10, at 3 to 10 C, and with ozone residuals between 0.3 to 2.0 mg/L, bacteriophage MS-2 (a surrogate test organism) and Hepatitis A virus were completely inactivated. Inactivations ranged from >3.9-log to >6-log, and occurred within very short contact periods (i.e., 5 seconds). A 1992 research report describes treatment studies conducted on MS-2, poliovirus, and Giardia cysts. It found that MS-2 in natural waters are very sensitive to ozone in comparison to poliovirus type 3. In addition, Giardia muris and enteric viruses may be inactivated by ozone (as the primary disinfectant) with 5 minutes contact time and ozone residuals of 0.5 to 0.6 mg/L to 3-log and 4-log removals, respectively. The report concludes that design of ozone as a primary treatment should be based on simple criteria including ozone residual, competing ozone demands, and a minimum contact time to meet the required cyst and viral inactivation requirements, in combination with USEPA guidance recommendations. Viral inactivation CT values for ozone were published in the original USEPA guidance manual for the SWTR.
The EPA has reviewed survey data submitted by the International Ozone Association and found that ozonation has been applied at many drinking water treatment facilities in the U.S. with capacities greater than 100,000 gal/day and some smaller facilities, for disinfection as well as for other water treatment objectives. Applications at the smallest water system size category (i.e., systems serving <500) are not plentiful. However, ozonation technology for even the smallest public water system applications is available from a number of suppliers, and is found to be currently in use in relevant systems. Ozone treatment, therefore, is a listed technology for all categories of public water systems.
Ozone Small Potable Water Systems
Ozone, the strongest oxidant and disinfectant in commercial use has been employed in over 3,000 large scale municipal plants world-wide. In August 1997, and again in August 1998, the U.S. EPA identified ozone as a Small System Compliance Technology for existing National Primary Drinking Water Regulations related to revisions in the 1996 Safe Drinking Water Act. Survey data developed to support the inclusion of ozone as a “Compliance Technology” identified that over half of the more than 260 U.S. municipal ozone installations known to be operating in early 1998 are in systems treating less than 1 MGD (e.g., plants that serve less than 10,000 persons). An additional 363 community, non-community and single family ozone installations using ultraviolet generation and filtration process also were identified.
Ozone Treatment of Potable Water
Ozonation has been in continuous use in Nice, France since 1906, to ensure disinfection of mountain stream water. Because ozone is both the strongest oxidant and strongest disinfectant available for potable water treatment, this unique material can be utilized for a number of specific water treatment applications, including disinfection, taste and odor control, color removal, iron and manganese oxidation, H2S removal, nitrite and cyanide destruction, oxidation of many organics (e.g., phenols, some pesticides, some detergents), algae destruction and removal, and as a coagulant aid.
Even though ozone is the strongest chemical disinfectant available for water treatment, there are some refractory organics that it will not oxidize, or will oxidize too slowly to be of practical significance. In such cases, ozone can be combined with UV radiation and/or hydrogen peroxide to produce the hydroxyl free radical, HO*, which is an even stronger oxidant than is molecular Ozone, O3. Deliberate production of the hydroxyl free radical starting with ozone has been termed “Ozone Advanced Oxidation”. Some groundwaters that are contaminated with chlorinated organic solvents and some refractory hydrocarbons are being treated successfully by ozone advanced oxidation techniques.
Properties and Generation of Ozone
At ambient temperatures, ozone is an unstable gas, partially soluble in water (generally more soluble than oxygen). Due to its instability (it quickly reverts to oxygen), ozone cannot be produced at a central manufacturing site, bottled, shipped and stored prior to use. It must be generated and applied on-site, as it is required. This means the installation of an ozone production plant at its point of use B which for small systems can be inside or outside of an individual home.
Ozone is generated for commercial uses either by corona discharge or by ultraviolet radiation. By the UV technique, rather low concentrations of ozone (below 0.1 wt %) are generated, whereas by corona discharge, ozone concentrations in the range of 1 – 4.5 wt % are produced when dry air is fed to the ozone generator. When concentrated oxygen is used as the feed gas, gas phase ozone concentrations of up to 14 to 18% (by wt) can be produced on commercial scale. Since ozone is only partially soluble in water, once it has been generated it now must be contacted with water to be treated in such a manner as to maximize the transfer of ozone from the gas phase into water. For this purpose, many types of ozone contactors have been developed; all of which are effective for their designed water treatment purposes. However, as higher concentration ozone gas is employed, contacting system design becomes more critical due to the lower gas to liquid ratios. Also, the use of oxygen as the feed gas can result in oxygen super saturation of the treated water causing both operational problems in following treatment processes and aesthetic in the distribution system.
Ozone contacting system options include atmospheric tall tower or pressurized gas to liquid mass transfer processes. Fine bubble diffusers, static mixers or venturi injectors can be used to mix the gas with the water to be treated in either full flow or sidestream configurations. In many small systems, small in-line injectors and pressurized reaction vessels replace the huge concrete, 20-ft deep bubble diffuser tanks which are cost-effective in large scale systems.
Once dissolved in water, ozone now is available to act upon water contaminants to accomplish its intended purposes of disinfection and/or oxidation. At low pH levels (3-6, for example) the ozone is present primarily in its molecular form (O3). However, as the pH rises, the decomposition of ozone to produce the hydroxyl free radical (HO*) becomes increasingly rapid. At pH 7 about 50% of the ozone transferred into water produces HO*. At pH > 10, the conversion of molecular O3 to HO* is virtually instantaneous.
Engineering Aspects of Ozonation Systems
Because ozone is such a powerful oxidant/disinfectant, the trick to applying it to solve water treatment problems is to do so in a manner that is effective for water treatment, yet at the same time ensuring the safety of people in the vicinity. Ozone safety issues are handled quite easily by use of proper ambient ozone monitoring, tank venting and ozone destruction. In the case of systems driven solely by a pumping/injector system, Ozone may be produced under vacuum, which ensures no leakage of Ozone into the operating environment.
The five basic components of an Ozone system include 1. Gas Preparation – either drying gas to a suitable dewpoint or using oxygen concentrators. 2. A suitable electrical power supply. 3. A properly sized Ozone Generator(s) 4. An Ozone contacting system. 5. Ozone off-gas destruction or suitable venting system.For corona discharge ozone generation, it is critical to feed the generator a clean and dry oxygen- containing gas. Moisture in the feed gas causes two operating problems. First, the amount of ozone produced by application of a given electrical energy level is lowered as relative humidity rises. Consequently, it is usually cost-effective to dry the air to a recommended dew point of minus 65’C (-65’C = -76’F) or lower. Second, when ozone is generated using air in the presence of moisture, the small amount of nitrogen oxides react with the moisture to produce nitric acid. Moist gas condensation at the cooling/heat transfer surfaces produces the corrosive compound which can soon cause corrosion problems in the ozone generation equipment, with concomitant increases in equipment maintenance requirements. Because of the high oxidative qualities of gas-phase ozone and the chance of moisture from a failing feed gas unit, small system managers should take extra care to make certain that all components in the ozone generator, ozone supply line, ozone gas to liquid mass transfer equipment and the contact vessel are ozone-compatible.
For large scale ozonation systems, the equipment for cleaning and drying feed gases can become quite complex. For example, effective air drying can involve the multiple treatment steps of air filtration, compression, cooling, desiccation, and final filtration prior to passage into an operating corona discharge ozone generator. For small community systems, several commercial-grade air dryers and small oxygen generators are available, but these must be matched carefully to the specifications of the ozone generator .
The need for efficient ozone contacting has been discussed earlier, and the final necessity is a unit for destruction of excess ozone always present in contactor off-gases when generated by corona discharge. Absent an effective ozone off-gas destruct unit, this excess ozone would be present for people in the vicinity to breathe, which is not recommended due to its strong oxidizing nature. Additionally, ozone is heavier than ambient air, can settle in the vicinity, and can attack oxidizable materials. Destruction of contactor off-gas ozone is readily accomplished thermally (370’C), catalytically, thermal-catalytically, or (only for small air-fed systems containing very low ozone concentrations) by passage through granular activated carbon. Care should be exercised in selecting an ozone destruct method whenever very high concentrations of ozone will be encountered.
To the five-component system outlined above can be added instrumentation and controls for ensuring the effective and safe operation of the total system. And now the concern for applying ozone to small water treatment systems becomes one of how to miniaturize the tried and true large scale units to be effective and affordable systems for treating water in small systems. Aside from simply making each of the five components smaller in physical size, there are some additional techniques for corner-cutting without sacrificing quality in terms of production of ozone at desirable gas-phase concentrations. For electrical power, the home or business wall plug providing 110-V or 220-V single phase power replaces 3-phase supplies at 230, 460 or 575- V required at large installations.
For air drying, desiccation or oxygen concentration is appropriate as the sole feed gas approach on small scale, replacing the multiple-treatments required at larger installations. For contacting, small in-line injectors replace the huge concrete, 20-ft deep bubble diffusers, which are cost-effective on large scale. In many small applications with extended storage capacity for prolonged ozone addition, UV generation of ozone can be practical for oxidation of iron and manganese, whereas UV generation at large water treatment plants is prohibitively higher in cost than corona discharge. Oxygen concentrators often replace air desiccation units to feed oxygen-enriched air to the ozone generators, thus producing higher gas phase ozone concentrations and increased output (g/h) per unit size on small scale, thus avoiding the need for on-site oxygen production and/or storage facilities.